Semiconductor structure with multiple transistors having various threshold voltages

ABSTRACT

A semiconductor structure includes first, second, and third transistor elements each having a first screening region concurrently formed therein. A second screening region is formed in the second and third transistor elements such that there is at least one characteristic of the screening region in the second transistor element that is different than the second screening region in the third transistor element. Different characteristics include doping concentration and depth of implant. In addition, a different characteristic may be achieved by concurrently implanting the second screening region in the second and third transistor element followed by implanting an additional dopant into the second screening region of the third transistor element.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 13/926,555filed Jun. 25, 2013 which claims the benefit of U.S. ProvisionalApplication No. 61/665,113 filed Jun. 27, 2012.

TECHNICAL FIELD

The following disclosure relates in general to semiconductor devices andprocessing and more particularly to a semiconductor structure withmultiple transistor elements having various threshold voltages andmethod of fabrication thereof.

BACKGROUND

Many integrated circuit designs use a variety of cells that performspecific functions. Integrated circuits can include logic, memory,controller and other functional blocks. Semiconductor integratedcircuits are fabricated in a semiconductor process, often using a CMOSprocess. Transistors are formed in a semiconductor substrate, andusually involve a sequence of fabrication steps that result in a gatewith adjacent source and drain, the source and drain being formed in achannel. A key setting for a transistor is the threshold voltage, whichin turn determines the voltage at which a transistor can be switched.Low threshold voltage devices are generally used for high speedcircuits, though low threshold voltage devices tend to have higherleakage power. High threshold voltage devices tend to result in slowerspeeds but are usually implemented when power reduction is desired. Itis generally known that variation in threshold voltage from the devicespecification is undesirable. Threshold voltage can be set byincorporating dopants into the transistor channel, either by way ofdirect channel implantation adjacent the gate oxide or by way of pocketor halo implants adjacent the source and drain. Such channel doping orhalo implants also have the positive effect of reducing short channeleffects especially as the gate length shrinks. Threshold voltagevariation can increase with scaling, however, because of random dopantfluctuations in the implanted channel area. The variation problemworsens as critical dimensions shrink because of the greater impact ofdopant fluctuations as the volume of the channel becomes smaller. As aresult, circuit design has become more limited over time in that circuitdesigners must account for greater potential variation in the deviceswith smaller gate dimensions, thus making it impossible to designcircuits with the technical freedom needed to build new and improvedsemiconductor chips. While CMOS technology has improved to allowcontinued scaling down of critical dimension, the associated and desiredscaling down of voltage has not followed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, wherein likereference numeral represent like parts, in which:

FIG. 1 shows an embodiment of a Deeply Depleted Channel (DDC) transistor100 having an enhanced body coefficient, along with the ability to setthreshold voltage Vt with enhanced precision;

FIGS. 2A-2C illustrate the dopant profiles for exemplary screeningregions for three different transistor device types constructed on acommon substrate;

FIGS. 3A-3C illustrate representative structures of the transistordevice types corresponding to the dopant profiles of FIGS. 2A-2C;

FIGS. 4A-4C are graphs illustrating an alternative dopant profile forexemplary screening regions for three different transistor device typesconstructed on a common substrate;

FIGS. 5A-5C are graphs illustrating still another alternative scheme forsetting Vt across three types of transistors;

FIGS. 6A-6B illustrates the impact of the doses and implant energy usedto implant the second screening region dopant on the threshold voltageand the leakage current for a PMOS transistor;

FIG. 7 illustrates the combined effect of the implant energy and implantdose used to implant the second screening region dopant on the thresholdvoltage and leakage current for a PMOS transistor;

FIGS. 8A and 8B illustrate embodiments that advantageously use twodifferent dopant species for the two screening region implants used toform dual screening regions;

FIG. 9 illustrates a semiconductor wafer supporting multiple die;

FIG. 10 illustrates one embodiment of a portion of a DDC transistormanufacturing process;

FIGS. 11A-11D illustrate a dopant profile and corresponding structurefor a DDC transistor having dual antipunchthrough (APT) regions withsingle and dual screening regions respectively;

FIGS. 12A-12C illustrate threshold voltage as a function of gate lengthfor DDC transistors having single and dual APT regions formed usingdifferent implant conditions;

FIG. 13 illustrates the body coefficient for PMOS LVt transistors havingsingle and dual Sb APT regions;

FIG. 14 illustrates the body coefficient for NMOS LVt transistors havingsingle and dual boron (B) APT regions;

FIGS. 15A-15B illustrate that dual APT regions have an effect on bodycoefficient for PMOS DDC transistor devices for given screening regionconditions;

FIGS. 16A and 16B illustrate that dual APT regions can provide anenhanced body coefficient for NMOS transistor devices for givenscreening region conditions.

DETAILED DESCRIPTION

Transistors having improved threshold voltage variation and thereforeenabling the scaling of supply voltage are disclosed. Embodiments ofstructures and fabrication methods allowing for reliable setting ofthreshold voltage, and with improved mobility, transconductance, drivecurrent, strong body coefficient and reduced junction leakage areprovided. More specifically, embodiments of doping profiles to result indifferent Vt targets for the different transistor device types withoutthe use of pocket or halo implants or channel implantation adjacent thegate oxide are disclosed.

FIG. 1 shows an embodiment of a Deeply Depleted Channel (DDC) transistor100 having an enhanced body coefficient, along with the ability to setthreshold voltage Vt with enhanced precision. The DDC transistor 100includes a gate electrode 102, source 104, drain 106, and a gatedielectric 128 positioned over a substantially undoped channel 110.Lightly doped source and drain extensions (SDE) 132, positionedrespectively adjacent to source 104 and drain 106, extend toward eachother, setting the transistor channel length.

The exemplary DDC transistor 100 is shown as an N-channel transistorhaving a source 104 and drain 106 made of N-type dopant material, formedupon a substrate such as a P-type doped silicon substrate providing aP-well 114 formed on a substrate 116. In addition, the N-channel DDCtransistor in FIG. 1 includes a highly doped screening region 112 madeof P-type dopant material, and a threshold voltage set region 111 madeof P-type dopant material. Substantially undoped channel 110 ispreferably formed using epitaxially-grown silicon, using a processrecipe that is intended to result in undoped crystalline silicon.Although substantially undoped channel 110 may be referred to herein asthe “undoped channel”, it is understood that a minimum or baseline levelof dopants are present due to unavoidable introduction of some foreignmaterial during the otherwise intrinsic epitaxial process. As a generalmatter, the “undoped channel” preferably has a dopant concentration ofless than 5×10¹⁷ atoms/cm³ in some portions thereof. However, it isdesirable that at least a portion of the channel underlying the gateremains undoped in the final transistor structure and certain processsteps are chosen to achieve this configuration. An N-channel DDCtransistor is shown in FIG. 1. Similarly, a P-channel DDC transistor canbe achieved by interchanging N and P regions.

The features of DDC transistor 100 can result in various transistordevice types. Such transistor device types include, but are not limitedto: P-FETs, N-FETs, FETs tailored for digital or analog circuitapplications, high-voltage FETs, high/normal/low frequency FETs, FETsoptimized to work at distinct voltages or voltage ranges, low/high powerFETs, and low, regular, or high threshold voltage transistors (i.e. lowVt, regular Vt, or high Vt—also referred to as LVt, RVt, or HVt,respectively), etc. Transistor device types are usually distinguished byelectrical characteristics (e.g. threshold voltage, mobility,transconductance, linearity, noise, power), which in turn can lendthemselves to be suitable for a particular application (e.g., signalprocessing or data storage). Since a complex integrated circuit such as,for instance, a system on a chip (SoC) may include many differentcircuit blocks having different transistor device types to achieve thedesired circuit performance, it is desirable to use a transistorstructure that can be readily fabricated to result in the varioustransistor device types.

A process for forming a DDC transistor may begin with forming thescreening region 112. In certain embodiments, the screening region isformed by providing the substrate having the P-well 114 and implantingscreening region dopant material thereon. Other methods may be used toform the screening region such as in-situ doped epitaxial silicondeposition, or epitaxial silicon deposition followed by verticallydirected dopant implantation to result in a heavily doped regionembedded a vertical distance downward from gate 102. Preferably, thescreening region is positioned such that the top surface of thescreening region is located approximately at a distance of Lg/1.5 toLg/5 below the gate (where Lg is the gate length). The screening regionis preferably formed before STI (shallow trench isolation) formation.Boron (B), Indium (I), or other P-type materials may be used for P-typeNMOS screening region material, and arsenic (As), antimony (Sb) orphosphorous (P) and other N-type materials can be used for PMOSscreening region material. The screening region 112, which is consideredheavily doped, has a significant dopant concentration, which may rangebetween about 5×10¹⁸ to 1×10²⁰ dopant atoms/cm³. Generally, if thescreening region 112 dopant level is on the higher end of the range, thescreening region 112 can simultaneously function as the thresholdvoltage setting region.

Though exceptions may apply, as a general matter it is desirable to takemeasures to inhibit the upward migration of dopants from the screeningregion, and in any event, controlling the degree to which dopants maymigrate upward as a mechanism for controlling the threshold voltagesetting is desired. All process steps occurring after the placement ofscreening region dopants are preferably performed within a limitedthermal budget. Moreover, for those dopants that tend to migrate or forflexibility in using a higher temperature in subsequent processes, agermanium (Ge), carbon (C), or other dopant migration resistant layercan be incorporated above the screening region to reduce upwardmigration of dopants. The dopant migration resistant layer can be formedby way of ion implantation, in-situ doped epitaxial growth, or otherprocess.

An optional threshold voltage set region 111 is usually positioned abovethe screening region 112. The threshold voltage set region 111 can beeither in contact with, adjacent to, incorporated within, or verticallyoffset from the screening region. In certain embodiments, the thresholdvoltage set region 111 is formed by ion implantation into the screeningregion 112, delta doping, controlled in-situ deposition, or by atomiclayer deposition. In alternative embodiments, the threshold voltage setregion 111 can be formed by way of controlled outdiffusion of dopantmaterial from the screening region 112 into an undoped epitaxial layerusing a predetermined thermal cycling recipe. Preferably, the thresholdvoltage set region 111 is formed before the undoped epitaxial layer isformed, though exceptions may apply. The threshold voltage is designedby targeting a dopant concentration and thickness of the thresholdvoltage set region 111 suitable to achieve the threshold voltage desiredfor the device. Note that if the screening region 112 concentration issufficiently high, then the screening region 112 can function as thethreshold voltage setting region and a separate threshold voltagesetting region is not needed. Preferably, the threshold voltage setregion 111 is fabricated to be a defined distance below gate dielectric128, leaving a substantially undoped channel layer directly adjacent tothe gate dielectric 128. The dopant concentration for the thresholdvoltage set region 111 depends on the desired threshold voltage for thedevice, taking into account the location of the threshold voltage setregion 111 relative to the gate. Preferably, the threshold voltage setregion 111 has a dopant concentration between about 1×10¹⁸ dopantatoms/cm³ and about 1×10¹⁹ dopant atoms per cm³. Alternatively, thethreshold voltage set region 111 can be designed to have a dopantconcentration that is approximately one third to one half of theconcentration of dopants in the screening region 112.

The final layer of the channel is formed preferably by way of a blanketepitaxial silicon deposition, although selective epitaxial depositionmay be used. The channel 110 is structured above the screening region112 and threshold voltage set region 111, having a selected thicknesstailored to the electrical specifications of the device. The thicknessof the substantially undoped channel 110 usually ranges fromapproximately 5-25 nm with a thicker undoped channel 110 usually usedfor a lower Vt device. To achieve the desired undoped channel 110thickness, a thermal cycle may be used to cause an outdiffusion ofdopants from the screening region 112 into a portion of the epitaxiallayer to result in a threshold voltage setting region 111 for a givenundoped channel region 110 thickness. To control the degree ofoutdiffusion of dopants across a variety of device types, migrationresistant layers of C, Ge, or the like can be utilized in selecteddevices. By achieving a thickness of the threshold voltage region by wayof the ion implantation, in-situ epitaxial growth or other methods suchas thermal cycle to effect a controlled diffusion a distance upward intothe channel, different thicknesses of channel 110 may be achieved. Stillfurther methods for establishing different thicknesses of channel 110may include selective epitaxial growth or a selective etch back with orwithout a blanket epitaxial growth or other thickness reduction.Isolation structures are preferably formed after the channel 110 isformed, but isolation may also be formed beforehand, particularly ifselective epitaxy is used to form the channel 110.

The transistor 100 is completed by forming a gate electrode 102 whichmay be a polysilicon gate or a metal gate stack, as well as SDE 132,spacers 130, and source 104 and drain 106 structures using conventionalfabrication methods, with the caveat that the thermal budget bemaintained within a selected constraint to avoid unwanted migration ofdopants from the previously formed screening region 112 and thresholdvoltage setting region 111. Note that versions of transistor 100 can beimplemented in any process node using a variety of transistor structuralschemes including, in the more advanced nodes, using techniques to applystress or strain in the channel. In conventional field effecttransistors (FETs), the threshold voltage can be set by directlyimplanting a “threshold voltage implant” into the channel, raising thethreshold voltage to an acceptable level that reduces transistoroff-state leakage while still allowing speedy transistor switching. Thethreshold voltage implant generally results in dopants permeatingthrough the entire channel region. Alternatively, the threshold voltage(V_(t)) in conventional FETs can also be set by a technique variouslyknown as “halo” implants, high angle implants, or pocket implants. Suchimplants create a localized, graded dopant distribution near atransistor source and drain that extends a distance into the channel.Both halo implants and channel implants introduce dopants into thechannel, resulting in random fluctuations of dopants in the channelwhich in turn can affect the actual threshold voltage for the device.Such conventional threshold voltage setting methods result inundesirable threshold voltage variability between transistors and withintransistor arrays. Additionally, such conventional threshold voltagesetting methods decrease mobility and channel transconductance for thedevice.

The screening region 112 creates a strong body coefficient amenable forreceiving a body bias. A body tap 126 to the screening region 112 of theDDC transistor can be formed in order to provide further control ofthreshold voltage. The applied bias can be either reverse or forwardbiased, and can result in significant changes to threshold voltage. Biascan be static or dynamic, and can be applied to isolated transistors, orto groups of transistors that share a common well. Biasing can be staticto set threshold voltage at a fixed set point or dynamic to adjust tochanges in transistor operating conditions or requirements. Varioussuitable biasing techniques are disclosed in U.S. Pat. No. 8,273,617,the entirety of which is herein incorporated by reference.

Further examples of transistor structure and manufacture suitable foruse in DDC transistors are disclosed in U.S. patent application Ser. No.12/895,785 filed Sep. 30, 2010 titled “Advanced Transistors withThreshold Voltage Set Dopant Structures” by Lucian Shifren, et al., U.S.Pat. No. 8,421,162, U.S. patent application Ser. No. 12/971,884 filed onDec. 17, 2010 titled “Low Power Semiconductor Transistor Structure andMethod of Fabrication Thereof” by Lucian Shifren, et al., and U.S.patent application Ser. No. 12/971,955 filed on Dec. 17, 2010 titled“Transistor with Threshold Voltage Set Notch and Method of FabricationThereof” by Reza Arghavani, et al., the respective contents of which areincorporated by reference herein in their entirety.

Many integrated circuit designs benefit from the availability of avariety, or range of transistor device types that can be included inthose integrated circuits. The availability of multiple transistordevice types provides engineers with the resources to produce optimizedcircuit designs, as well as to produce circuit designs that mightotherwise be unachievable if limited to a small number of transistordevice types. As a practical matter, it is desirable that eachintegrated circuit on a wafer be able to incorporate all, or any subsetof, the range of transistor device types available in an integratedcircuit manufacturing process while achieving a limited variation inthreshold voltage both locally and globally. It is also desirable toreduce the off-state leakage current and to achieve a limited variationin the off-state leakage current for the range of transistor devicetypes available in the integrated circuit.

Various embodiments described below use a combination of ionimplantations to form dual screening regions to achieve differenttransistor device types. Dual screening regions are advantageously usedto provide different transistor device types in terms of thresholdvoltages while achieving a reduced off-state leakage current. Incomparison, a transistor device that uses a single screening region mayhave a similar threshold voltage but may have higher junction leakage.With dual screens, each peak screening region dopant concentration maybe reduced compared with the case of the dopant concentration of asingle screening region for a given threshold voltage. Additionally,dual antipunchthrough (APT) regions are disclosed. Dual APT can providea specified body coefficient using a lower peak concentration ascompared to the peak concentration of a single implant APT region for asubstantially similar body coefficient. Dual APT regions also providethe benefit of reducing the off-state leakage current of the differenttransistor device types, for instance if dual APT regions use acombination of a shallower and deeper APT region implants compared to amid-energy single APT region implant. Transistors having shallower APTregions (due to lower energy APT region implants) can typically includea lower peak screening region dopant concentration to achieve a targetthreshold voltage. The advantages of dual APT regions can be obtainedwhether with single screening regions or dual screening regions.

Typically, the value of the threshold voltage is related to theconcentration of dopants in the screening region. For variousembodiments described below, the concentration of dopants is illustratedas a function of depth (also referred to as a dopant profile), where thezero depth position typically approximates the position of the gateoxide in the device.

FIGS. 2A-2C illustrate the dopant profiles for exemplary screeningregions for three different transistor device types constructed on acommon substrate, in which the doped regions are separated from the gateby a substantially undoped semiconductor layer (preferably intrinsicsilicon having a dopant concentration of less than 5×10¹⁷ atoms/cm³). Arange of Vt's can be achieved thereby, for instance a low thresholdvoltage (LVt) 205, a regular threshold voltage (RVt) 210, and a highthreshold voltage (HVt) 215. The screening region dopant profile for theLVt transistor device type has a peak screening region dopantconcentration of CF1 atoms/cm³, the screening region dopant profile forthe RVt transistor device type has a peak screening region dopantconcentration of CF2 atoms/cm³, and the screening region dopant profilefor the HVt transistor has a peak screening region dopant concentrationof CF3 atoms/cm³, where CF1<CF2<CF3. Such dopant concentrations can beachieved by selected doses for the implants. In one embodiment, Sb isimplanted at a dose of 1×10¹³ atoms/cm², 2×10¹³ atoms/cm², and 3×10¹³atoms/cm² to form the screening regions of the LVt, RVt, and HVttransistor device types respectively. Implant energies in the range of10-50 keV can be used to implant the screening region dopants, where thelocation of the peak is generally related to the implant energy used.The deeper the peak desired, the higher the selected energy for theimplant. In the embodiment of FIGS. 2A-2C, the screening region dopantsfor the three transistor device types are implanted using the sameimplant energy but with different doses resulting in three screeningregion dopant profiles 205, 210, and 215 having three different peakconcentrations, where the peak is located at approximately the samedepth relative to the top surface of the substrate.

FIGS. 3A-3C illustrate representative structures of the transistordevice types corresponding to the dopant profiles of FIGS. 2A-2C,showing in cross-section how screening regions may appear. In FIG. 3A,there may be a screening region 305 placed a defined depth below gatestack 308 with an undoped channel 307 in the space between screeningregion 305 and gate stack 308 and having a defined thickness selected toachieve threshold voltage, junction leakage, and other devicecharacteristics. Preferably, the thickness of screening region 305 isabout 3 nm to 10 nm in thickness or more, but in any event is preferablyless thick than the gate length of gate stack 308. Source and drain pair306 are on either side of screening region 305 such that screeningregion 305 extends laterally across and underneath undoped channel 307and abutting the edges of source and drain pair 306. IN FIG. 3B, theremay be screening region 310 that is more heavily doped than screeningregion 305, placed a similar depth below gate stack 313 as screeningregion 305 to its gate stack 308, with undoped channel 307 formed usinga blanket epitaxial process so that it forms the same silicon thicknessfor the undoped channel for all of the devices having screening regions.Screening region 310 has a defined thickness selected to achievethreshold voltage, junction leakage, and other device characteristics.Preferably, the thickness of screening region 310 is about 3 nm to 15 nmin thickness and may be thicker than screening region 305, but in anyevent is preferably less thick than the gate length of gate stack 308.Source and drain pair 311 are on either side of screening region 310such that screening region 310 extends laterally across and underneathundoped channel 307 and abutting the edges of source and drain pair 311.IN FIG. 3C, there may be screening region 315 that is more heavily dopedthan screening region 310, placed a similar depth below gate stack 318as screening region 310 and 305 to their respective gate stacks 313 and308, with undoped channel 307 forming the space between screening region315 and gate stack 318. Screening region 315 has a defined thicknessselected to achieve threshold voltage, junction leakage, and otherdevice characteristics. Preferably, the thickness of screening region315 is about 3 nm to 20 nm in thickness and may be thicker thanscreening regions 310 and 305, but in any event is preferably less thickthan the gate length of gate stack 318. Source and drain pair 316 are oneither side of screening region 315 such that screening region 315extends laterally across and underneath undoped channel 307 and abuttingthe edges of source and drain pair 316. The transistors illustrated inFIGS. 3A-3C are to demonstrate exemplary schemes for placement of therespective screening regions 305, 310, and 315, though specificimplementations may differ depending on a variety of desiredcharacteristics for the devices in the context of the semiconductorfabrication node. For instance, source and drains may be elevated andfabricated using selective epitaxial growth using a silicon,silicon-germanium, or other material to form the source and drain or anyother process that imparts a stress in the channel.

Note that it may be desired to locate the screening regions at differentdepths to achieve different threshold voltage and other characteristicsfor the device. Screening region depth can be controlled based oncontrolling the process settings, for instance higher ion implant energyto drive the ions deeper or lower ion implant energy to maintain a moreshallow implanted region. After the screening region dopants areemplaced, the channel is completed by depositing an epitaxial siliconlayer on the substrate over the screening region dopants. It followsthat, if the screening region dopants are at the approximately samedepth below the top surface of the substrate, then to achieve differingVt's, different implant doses are used to modulate the Vt value. Ahigher implant dose generally results in a higher concentration ofdopants. A lower implant dose generally results in a lesserconcentration of dopants. If the screening region dopant implant processuses differing energies, then the Vt values will be modulated based uponthe different depths of the screens or, put another way, based upon thedifferent resulting relative thicknesses of the undoped epitaxial layer.

FIGS. 4A-4C are graphs illustrating an alternative dopant profile forexemplary screening regions for three different transistor device typesconstructed on a common substrate. Note that the profiles may representthe distribution of dopant material prior to anneal. Post anneal, theprofiles may be less distinct. Preferably, the profiles are achieved byway of separate ion implant steps. The common substrate is doped tocreate different transistor device types, e.g. a low threshold voltage(LVt), a regular threshold voltage (RVt), and a high threshold voltage(HVt) transistor corresponding to FIGS. 4A-4C respectively. Preferably,the screening region dopant profiles for the three transistor devicetypes illustrated in FIGS. 4A-4C are obtained by performing multiplescreening region dopant implants. In one embodiment, a first screeningregion dopant can be implanted for all three devices as shown herein asdopant profiles 405, 410, and 415 for the LVt, RVt, and HVt transistordevice types respectively. An additional implant step is performed forthe RVt transistor device type to form the second screening regiondopant profile 420, such that the combination of the dopant profiles 410and 420 sets the threshold voltage of the RVt transistor device type. Anadditional implant step is performed for the HVt transistor device typeto form the second screening region dopant profile 425, where thecombination of the dopant profiles 415 and 425 sets the thresholdvoltage of the HVt transistor device type. The advantage of the schemeillustrated in FIGS. 4A-4C is that a reduced dose implant can be used toachieve a peak concentration in the low end of the screening regionrange and, if desired, the same dose and energy for the screening regionfor the LVt device can be used for the RVt and HVt devices. Instead ofrelying upon a higher peak concentration for the screening region forthe RVt and HVt devices, a reduced peak concentration is used,preferably at approximately the same depth within the substrate for eachof the devices, and the Vt is achieved by implanting a secondary dopantprofile at a location adjacent to but closer to the gate using a doseselected to result in the peak concentration appropriate to set the Vtfor the device. As illustrated in FIGS. 4A-4C, the LVt device does notcontain a secondary implant and uses the initial screening regionimplant at peak concentration CF1, the RVt device includes the screeningregion implant at peak concentration CF1 and contains a secondaryimplant at concentration CS1, and the HVt device includes the screeningregion implant at peak concentration CF1 and contains a secondaryimplant at concentration CS2 which is higher than CS1. All peakconcentrations of dopants and relative depths within the substrate aredetermined as part of the device design to achieve the desired Vt whilecomprehending other design constraints including leakage, drive-current,and other factors understood by those skilled in the art.

FIGS. 5A-5C are graphs illustrating still another alternative scheme forsetting Vt across three types of transistors. The common substrate isdoped to create dopant profiles for different transistor device types,e.g. a low threshold voltage (LVt), a regular threshold voltage (RVt),and a high threshold voltage (HVt) transistor. Preferably, a firstscreening region dopant is implanted for all three transistor devicetypes resulting in first screening region dopant profiles 505, 510, and515 for the LVt, RVt, and HVt transistor device types respectively.First screening region dopant profiles 505, 510, and 515 are preferablyformed to have approximately the same peak concentration, designated asCF, at approximately the same depth within the substrate. An additionalscreening region dopant implant step is performed for the RVt transistordevice type to form the second screening region dopant profile 520 at apeak concentration of CS, where the combination of the dopant profiles510 and 520 sets the threshold voltage of the RVt transistor devicetype. An additional screening region dopant implant step is performedfor the HVt transistor device type to form the second screening regiondopant profile 525, using a reduced energy with the same orapproximately the same dose so that second screening region dopantprofile 525 is at peak concentration of approximately CS but offset fromthe location of screening region dopant profile 515 to be located closerto the gate. Dopant profiles 515 and 525 may be separate from each otherbut connected by a lesser amount of dopant concentration as shown,namely a valley between the two peaks, or the dopant profiles 515 and525 may be isolated from each other. In effect, the undoped channel forthe device at FIG. 5C is thinner than for FIG. 5B and FIG. 5A. Thecombination of the dopant profiles 515 and 525 sets the thresholdvoltage of the HVt transistor device type. The second screening regionimplant for the RVt and HVt transistor device types can be performedusing the same dopant species at a substantially similar dose but usingdifferent implant energies, such that the peak dopant concentrations ofthe second dopant profile is approximately the same, but the peak ispositioned at a different depth for the two transistor device types. Inalternative embodiments, a combination of different dopant species,different dopant doses, and different implant energies can be used toimplant the second dopant to form the screening regions of the differenttransistor device types. All peak concentrations of dopants and relativedepths within the substrate are determined as part of the device designto achieve the desired Vt while comprehending other design constraintsincluding leakage, drive-current, and other factors understood by thoseskilled in the art.

FIG. 6A illustrates the impact 600 of the doses used to implant thesecond screening region dopant on the threshold voltage and the leakagecurrent for a PMOS transistor. The two graphs 605 and 610 in FIG. 6A areobtained from TCAD simulations performed for a PMOS DDC transistorhaving a single implant screening region and dual screening regionsrespectively. Graph 605 illustrates the leakage current Isub as afunction of threshold voltage for a PMOS transistor having only onescreening region implant consisting of Sb implanted at 20 keV usingdoses in the range of 1×10¹³ to 2×10¹³ atoms/cm². Point 605A of graph605 corresponds to a dose of 1×10¹³ atoms/cm² and point 605B of thegraph 605 corresponds to a dose of 2×10¹³ atoms/cm². Graph 610illustrates the leakage current Isub as a function of threshold voltagefor a PMOS transistor having dual screening regions, where the firstscreening region implant is Sb implanted at 20 keV using doses in therange of 1×10¹³ to 2×10¹³ atoms/cm² and the second screening regionimplant is Sb implanted at 10 keV using doses in the range of 2×10¹² to5×10¹² atoms/cm². Point 610A corresponds to a dose of 2×10¹² atoms/cm²,point 610B corresponds to a dose of 3×10¹² atoms/cm², point 610Ccorresponds to a dose of 4×10¹² atoms/cm², and point 610D corresponds toa dose 5×10¹² atoms/cm².

FIG. 6B illustrates the impact 601 of the implant energy used to implantthe second screening region dopant on the threshold voltage and theleakage current for a PMOS transistor. The three graphs 615, 620, and625 in FIG. 6B are obtained from TCAD simulations performed for a PMOSDDC transistor having a single screening region and dual screeningregions respectively, where different implant energies are used toimplant the second screening region dopant. Graph 615 illustrates theleakage current Isub as a function of threshold voltage for a PMOStransistor having only one screening region implant consisting of Sbimplanted at 40 keV using doses in the range of 1×10¹³ to 2×10¹³atoms/cm². Point 615A of graph 615 corresponds to a dose of 1×10¹³atoms/cm² and point 615B of graph 615 corresponds to a dose of 2×10¹³atoms/cm². Graph 620 illustrates the leakage current Isub as a functionof threshold voltage for a PMOS transistor having dual screeningregions, where the first screening region implant is Sb implanted at 40keV using doses in the range of 1×10¹³ to 2×10¹³ atoms/cm² and thesecond screening region implant is Sb implanted at 20 keV using doses inthe range of 0.5×10¹³ to 1×10¹³ atoms/cm². Point 620A corresponds to adose of 0.5×10¹³ atoms/cm², point 620B corresponds to a dose of 0.6×10¹³atoms/cm², and point 620C corresponds to a dose of 1×10¹³ atoms/cm².Graph 625 illustrates the leakage current Isub as a function ofthreshold voltage for a PMOS transistor having dual screening regions,where the first screening region implant is Sb implanted at 40 keV usingdoses in the range of 1×10¹³ to 2×10¹³ atoms/cm² and the secondscreening region implant is Sb implanted at 10 keV using doses in therange of 0.5×10¹³ to 0.6×10¹³ atoms/cm². Point 625A corresponds to adose of 0.5×10¹³ atoms/cm² and point 625B corresponds to a dose of0.6×10¹³ atoms/cm².

FIG. 7 illustrates the combined effect 700 of the implant energy andimplant dose used to implant the second screening region dopant on thethreshold voltage and leakage current for a PMOS transistor. FIG. 7includes an overlay of graphs 605 and 610 (FIG. 6A) and graphs 615, 620,and 625 (FIG. 6B). It is noted that a 200 mV range of threshold voltage,as indicated by the interval 705, can be obtained at substantially thesame leakage by appropriate selection of the first screening regionimplant and the second screening region implant conditions. It is alsonoted that the graphs 610 and 625 have substantially similar slope andthat both graphs show substantially similar threshold voltage andleakage current for the same second screening region implant dose.Therefore, the implant conditions of the second screening region dopantcan have a dominant effect on setting the threshold voltage.

The dual screening regions described above can be formed by implantingeither the same dopant species for the first screening region implant ora different dopant species can be used for the second screening regionimplant, wherein the dopant species are of the same polarity. FIGS. 8Aand 8B illustrate embodiments that advantageously use two differentdopant species for the two screening region implants used to form thedual screening regions. FIG. 8A illustrates an embodiment, where thesecond dopant species is a heavier molecule than the first dopantspecies and, therefore, the second dopant species can be implanted usinga higher implant energy to form a shallow second implant for thescreening region as compared to the implant energy that would berequired if the first dopant species were used to form the shallowsecond implant. In FIG. 8A, dual screening regions are formed for anNMOS transistor by implanting B to form the first implant and using BF2to form the second implant. Since BF2 is approximately five timesheavier than B, the implant energy used for the BF2 implant can be fivetimes the implant energy that would be used to implant B. This isadvantageous because the high energy implant can be more preciselycontrolled. FIG. 8B illustrates an embodiment where the first and seconddopant species diffuse at different rates during dopant activationanneal. In FIG. 8B, dual screening regions are formed for a PMOStransistor by implanting Sb to form the first implant and using As toform the second implant, where the Sb and As implant energies and dosesare selected to form the Sb and As doped implants at approximately thesame depth. However, during subsequent thermal processing such asactivation anneal, As will diffuse more than Sb and, therefore, forms ashallow doped region as illustrated in FIG. 8B. The use of a seconddopant species that diffuses more also permits higher dopant energies tobe used to implant the second dopant species.

FIG. 9 illustrates a semiconductor wafer 942 supporting multiple diesuch as previously described. In accordance with the present disclosure,each die can support multiple blocks of circuitry, each block having oneor more transistor types. Such an arrangement enables the creation ofcomplex system on a chip (SoC), integrated circuits, or similar die thatoptionally include FETs tailored for analog or digital circuitapplications, along with improved transistors such as DDC transistors.For example, four useful blocks in a single die are illustrated asfollows. Block 944 outlines a collection of deeply depleted channel(DDC) transistors having low threshold voltage, block 945 outlines acollection of DDC transistors having regular threshold voltage, block946 outlines a collection of DDC transistors having high thresholdvoltage, and block 947 outlines a collection of DDC transistors tailoredfor a static random access memory cell. As will be appreciated, thesetransistor types are representative and not intended to limit thetransistor device types that can be usefully formed on a die or wafer.Wafer 900 includes a substrate 902 (typically silicon) that can beimplanted with optional APT regions and required single or dualscreening regions 904 and an epitaxial blanket layer 906 formed afterimplantation of dopants in screening region 904. Wafer 900 can alsoinclude an optional threshold voltage set region (not shown in FIG. 9)positioned between the screening region 904 and the epitaxial blanketlayer 906.

FIG. 10 illustrates one embodiment of a portion of a DDC transistormanufacturing process 1000. A semiconductor wafer is masked at step 1002with a “zero layer” alignment mask to define dopant implantable wellregions. To illustrate one embodiment, it is shown in FIG. 10 to createPMOS dopant structures followed by NMOS dopant structures, but inimplementation the order can be reversed. In FIG. 10, a deep N-well canbe optionally formed at step 1004 in combination with or alternative toa conventional N-well. A first screening region dopant is implanted atstep 1006 to form a first highly doped screening region for the LVt,RVt, and HVt PMOS transistor device types. Typically, implant conditionsfor the first screening region dopant are selected to provide the targetthreshold voltage for the PMOS LVt transistor device type. At step 1008,the PMOS LVt and HVt devices are masked and an RVt additional screeningregion dopant is implanted to form dual screening regions for the RVtPMOS transistors. The implant conditions for the additional RVtscreening region dopant are selected such that the combination of thefirst screening region dopant and the additional RVt screening regiondopant provide the target threshold voltage for the PMOS RVt device. Atstep 1010, the PMOS LVt and RVt devices are masked and an additional HVtscreening region dopant is implanted to form dual screening regions forthe HVt PMOS transistors. The implant conditions for the additional HVtscreening region dopant are selected such that the combination of thefirst screening region dopant and the additional HVt screening regiondopant provide the target threshold voltage for the PMOS HVt device. Inalternative embodiments, the additional RVt screening region dopant isimplanted as part of the dual screening regions for both the PMOS RVtand HVt devices and both LVt and RVt devices are then masked to allowfor a still further HVt screening region implant for the HVt devicesonly. For this embodiment, the implant condition for the first screeningregion dopant is selected to provide the target threshold voltage forthe PMOS LVt devices, the implant conditions of the additional RVt andthe additional HVt dopants are selected such the combination of thefirst screening region dopant and the additional RVt dopant provides thetarget threshold voltage for the RVt devices, and the combination of allthree screening region dopants (i.e., the first screening region dopant,the additional RVt dopant, and the additional HVt dopant) provides thetarget threshold voltage for the HVt devices. Other well implants suchas the APT region implant can be formed in the N-well before or afterimplanting the screening region dopants in steps 1006, 1008, and 1010.

After masking the N-well, the P-well is implanted at step 1012. A firstscreening region dopant is implanted at step 1014 to form a first highlydoped screening region for the LVt, RVt, and HVt NMOS transistor devicetypes. Typically, implant conditions for the first screening regiondopant are selected to provide the target threshold voltage for the NMOSLVt transistor device type. At step 1016, the NMOS LVt and HVt devicesare masked and an additional RVt screening region dopant is implanted toform dual screening regions for the RVt NMOS transistors. The implantconditions for the additional RVt screening region dopant are selectedsuch that the combination of the first screening region dopant and theadditional RVt screening region dopant provide the target thresholdvoltage for the NMOS RVt device. At step 1018, the NMOS LVt and RVtdevices are masked and an additional HVt screening region dopant isimplanted to form dual screening regions for the HVt NMOS transistors.The implant conditions for the additional HVt screening region dopantare selected such that the combination of the first screening regiondopant and the additional HVt screening region provide the targetthreshold voltage for the NMOS HVt device. In alternative embodiments,the additional RVt screening region dopant is implanted as part of thedual screening regions for both the NMOS RVt and HVt devices and theNMOS LVt and RVt devices are then masked to allow for a still furtherscreening region implant for the NMOS HVt devices only. For thisembodiment, the implant condition for the first screening region dopantis selected to provide the target threshold voltage for the NMOS LVtdevices, the implant conditions of the additional RVt and the additionalHVt dopants are selected such the combination of the first screeningregion dopant and the additional RVt dopant provides the targetthreshold voltage for the NMOS RVt devices, and the combination of allthree screening region dopants (i.e., the first screening region dopant,the additional RVt dopant, and the additional HVt dopant) provides thetarget threshold voltage for the NMOS HVt devices. Other well implantssuch as the APT region implant can be formed in the P-well before orafter implanting the screening region dopants in steps 1014, 1016, and1018.

Next, at step 1020, a capping silicon epitaxial layer is deposited/grownacross the entire substrate using a process that does not include addeddopant species so that the resulting channel is substantially undopedand is of a resulting thickness tailored to achieve the multitude ofthreshold voltages. Typically the epitaxial layer is 100% intrinsicsilicon, but silicon germanium or other non-silicon in-situ depositedatoms can also be added to the epitaxial layer either across thesubstrate or a preselected device location using masks, thoughpreferably the resulting material from the epitaxial growth process isintrinsic in terms of dopant-based polarity. For further adjustment ofVt, a thermal cycling can be used to cause a controlled outdiffusion ofsome of the screening region dopants. Following epitaxial growth, atstep 1022, shallow trench isolation (STI) structures are formed. Insteps 1024 and 1026, gate structures, spacers, contacts, stressimplants, tensile films, dielectric coatings, and the like are thenformed to establish structures for operable transistors. The processesused to form the various structures are generally conventional, thoughwithin a defined thermal cycle and with appropriate adjustments toconventional process recipes to comprehend reduced temperatures fromotherwise high-temperature steps. In some devices, optionally,additional channel doping can be done using halo implants and/ortraditional channel implants to render such devices conventional asopposed to DDC. It shall further be noted that the exemplary dopantprofiles can be achieved using alternative processes. Although theprocess sequence of doping the screening region followed by forming theepitaxial undoped layer may be preferred, other processes can be used,for instance providing an undoped semiconductor region and thenperforming ion implantation at selected higher energies to drive thedopants down a depth through the undoped semiconductor region to achievethe exemplary dopant profiles. A further alternative process is toreplace ion implantation with in-situ doped epitaxial growth to achievethe doped screening regions followed by deposition of semiconductormaterial to create the desired dopant profiles having the screeningregions embedded a depth below the gate.

FIG. 11A illustrates a dopant profile 1100 for a DDC transistor havingdual APT regions and a single screening region. FIG. 11B illustrates astructure 1120 with the dual APT regions 1105 and 1110 and the singlescreening region 1115 underlying an undoped channel 1102. The dopantprofile 1100 includes two APT region implants having dopant profilesthat form the dual APT regions 1105 and 1110. The dopant profile 1100also includes a single screening region implant having a dopant profilethat form the single screening region 1115. Typically, the peak dopantconcentration of the APT region implant positioned closest to thescreening region, i.e. the dopant profile for APT region 1105, isgreater than the peak dopant concentration of the APT region implant forAPT region 1110 that is positioned deeper in the substrate. However, inalternative embodiments, the peak dopant concentration of the dopantprofiles for APT region 1105 and APT region 1110 can be approximatelythe same. Though shown as adjacent and in contact with one another, APTregions 1105 and 1110 and screening region 1115 may be spaced apart fromeach other as desired.

FIG. 11C illustrates a dopant profile 1150 for a DDC transistor havingdual APT regions and dual screening regions. FIG. 11D illustrates astructure 1180 with the dual APT regions 1155 and 1160 and the dualscreening regions 1165 and 1170 underlying an undoped channel 1182. Thedopant profile 1150 includes two APT region implants having dopantprofiles that form the dual APT regions 1155 and 1160. The dopantprofile 1150 also includes two screening region implants having a dopantprofile that form dual screening regions 1165 and 1170. The dual APTregions 1155 and 1160 can be combined with a single or dual (or tripleor more) screening regions, in combinations of varying peak dopantconcentrations, in embodiments that are not shown herein. Properselection of the dual APT region dose and energy condition allows theAPT region to perform its primary function of preventing deeppunchthrough between the source and drain regions (which would pinch-offthe screening region isolating the screening region from the body biasvoltage) while minimizing the junction leakage that can be caused byexcessive screening region and APT region implant dose. A single APTregion implant controls the pinch-off performance through increased doseat a penalty of higher junction leakage from the increased APT regionpeak concentration. A wider APT region made from two separatelyoptimized implants can be even more effective at protecting againstpinch-off than a single APT region implant and allows the peakconcentration for each implant to be lower than the equivalent singleAPT region implant resulting in overall lower leakage. Though shown asadjacent and in contact with one another, APT regions 1155 and 1160 andscreening regions 1165 and 1170 may be spaced apart from each other asdesired.

For the dual APT region dopant profiles illustrated in FIGS. 11A and11C, the deep APT region implant corresponding to the dopant profiles ofAPT regions 1110 and 1160 can assist the respective screening regions bycontrolling the depletion region generated by the operational voltageand, therefore, preventing the respective screening regions from beingpinched off by the depletion region. Preventing the pinch off of thescreening regions allows the screening regions to be biased by a bodybias voltage applied to the transistor body. Typically the peakconcentration of the deep APT region implant (i.e. dopant profiles ofAPT regions 1110 and 1160) is selected based on a predetermined range ofbody bias voltages to be applied to the DDC transistor such that theselected peak concentration prevents screening region pinch off for thepredetermined range of body bias voltages. Typically the peak dopantconcentration of the shallow APT region implant (i.e. dopant profilesfor APT regions 1105 and 1155) is selected to be lower than the peakscreening region dopant concentration.

One of the advantages of using dual APT regions is that the lower peakdopant concentration in the dual APT region structure as compared tothat of a single APT region helps to reduce junction leakage that mayotherwise be present in a DDC device. Further, when dual APT regions areused, the device can more readily be designed with a reduced peakconcentration screening region, either as a single screening region ordual screening regions, which provides advantages of reduced junctionleakage. Having two implanted APT regions more readily allows for acontinuum of doping extending from the screening region down through thedevice to the well. In contrast, a single implanted APT region generallyhas a tighter Gaussian distribution. The tighter Gaussian distributionmakes for a potential pocket of very low-doped area between thescreening region and the single APT region. Such a pocket that is verylow in dopants essentially separates the screening region from the APTregion, rendering the APT region less effective. The dual APT regionscan also be combined with diffusion mitigation techniques, for instanceGe preamorphization implants (PAI) with carbon implants. With diffusionmitigation techniques, a selected target APT region dopant profile canbe achieved using lower implant doses to form wider implanted regiondopant profiles as a starting point, because the implanted APT regiondopants are less apt to diffuse and spread during subsequent thermalsteps.

FIGS. 12A-12C illustrate threshold voltage as a function of gate lengthfor DDC transistors having single and dual APT regions formed usingdifferent implant conditions. FIG. 12A illustrates the changes inthreshold voltage as a function of drawn channel length for a transistorhaving a drawn width of 1 μm at a body bias voltage of 0.3 volts. FIG.12B illustrates the difference between the threshold voltage at a bodybias voltage of 0 volts and at a body bias voltage of 0.9 volts as afunction of drawn channel length for a DDC transistor having a drawnchannel width of 1 μm. FIG. 12C is an expanded version of a portion ofthe curves illustrated in FIG. 12B. FIGS. 12B and 12C provide a measureof the changes in the body coefficient as a function of the drawnchannel length. The threshold voltages corresponding to four differentAPT region implant conditions are illustrated in these figures—(i)single implant APT region formed by implanting Sb at 160 keV using adose of 0.9×10¹³ atoms/cm², (ii) single implant APT region formed byimplanting Sb at 130 keV using a dose of 0.9×10¹³ atoms/cm², (iii)single implant APT region formed by implanting Sb at 130 keV using adose of 1.2×10¹³ atoms/cm², and (iv) dual APT regions formed by a firstimplant of Sb at 130 keV using a dose of 0.6×10¹³ atoms/cm² and a secondimplant of Sb at 80 keV using a dose of 1.2×10¹³ atoms/cm². It is notedfrom FIGS. 12A-12C that the lowest Vt roll-off is obtained for the dualAPT regions. FIGS. 12A-12C show that dual APT regions can result inlower threshold voltage roll-off (Vt roll off).

FIG. 13 illustrates the body coefficient for PMOS LVt transistors havingsingle and dual Sb APT regions. It is noted that the PMOS transistorshaving dual APT regions have a higher body coefficient at a body biasvoltage of −0.3 V (labeled Body Factor) as compared to the PMOStransistors having a single implant APT region.

FIG. 14 illustrates the body coefficient for NMOS LVt transistors havingsingle and dual boron (B) APT regions. It is noted that the NMOStransistors having dual APT regions have a higher body coefficient at abody bias voltage of 0.3 V (labeled Body Factor) as compared to the NMOStransistors having a single implant APT region.

FIGS. 15A and 15B illustrate that dual APT regions have an effect onbody coefficient for NMOS DDC transistor devices for given screeningregion conditions.

FIGS. 15A and 15B illustrate the median threshold voltage for NMOStransistors of various widths as a function of four different appliedbody bias voltages—0 volts, 0.3 volts, 0.6 volts, and 0.9 volts. It isnoted that for all channel widths, the NMOS transistors having dual APTregions have a higher body coefficient compared to NMOS transistors withsingle APT regions as indicated by the threshold voltage response forvarying applied body bias voltage. In addition, the threshold voltage ofdual APT NMOS transistors varies less in response to the applied bodybias voltage for smaller channel widths, indicating an improved narrow-Zeffect in the DDC devices having the dual APT regions.

FIGS. 16A and 16B illustrate that dual APT regions can provide anenhanced body coefficient for PMOS transistor devices for givenscreening region conditions. FIGS. 16A and 16B illustrate the medianthreshold voltage for PMOS transistors of various widths as a functionof four different applied body bias voltages—0 volts, 0.3 volts, 0.6volts, and 0.9 volts. It is noted that for all channel widths the PMOStransistors having dual APT regions have a higher body coefficientcompared to PMOS transistors with single APT regions as indicated by thethreshold voltage response for varying applied body bias voltage. Inaddition, the threshold voltage of dual APT PMOS transistors varies lessin response to the applied body bias voltage for smaller channel widths,indicating an improved narrow-Z effect in the DDC devices having thedual APT regions.

In one embodiment, a target LVt transistor device type having a targetthreshold voltage of 0.38 V can be achieved using screening regionimplant dose of 5×10¹² atoms/cm² for a transistor using dual APT regions(where the dual APT regions are formed with a first Sb implant at 80 keVusing a dose of 1.2×10¹³ atoms/cm² and a second Sb implant at 130 keVusing a dose of 1.2×10¹³ atoms/cm²) as compared to a higher screeningregion dose of 8×10¹² atoms/cm² for a transistor using a single APTregion (where the single APT region is formed with an Sb implant at 130keV using a dose of 1.2×10¹³ atoms/cm²). In addition, the body factor ofthe dual APT LVt transistor is higher compared to that of the single APTLVt transistor, 85 as compared to 60 respectively, where the body factoris measured at a body bias voltage of −0.3 V. In an alternativeembodiment, a target SVt transistor device type having a targetthreshold voltage of 0.46 V can be achieved using a screening regionimplant dose of 1.2×10¹³ atoms/cm² for a transistor using dual APTregions (where the dual APT regions are formed with a first Sb implantat 80 keV using a dose of 1.2×10¹³ atoms/cm² and a second Sb implant at130 keV using a dose of 1.2×10¹³ atoms/cm²) as compared to a higherscreening region dose of 1.4×10¹³ atoms/cm² for a transistor using asingle APT region (where the single APT region is formed with an Sbimplant at 130 keV using a dose of 1.2×10¹³ atoms/cm²). The body factorof the dual APT SVt transistor is also higher compared to that of thesingle APT SVt transistor, 96 as compared to 85 respectively, where thebody factor is measured at a body bias voltage of −0.3 V.

Transistors created according to the foregoing embodiments, structures,and processes can be formed on the die alone or in combination withother transistor types. Transistors formed according to the disclosedstructures and processes can have a reduced mismatch arising fromscattered or random dopant variations as compared to conventional MOSanalog or digital transistors. This is particularly important fortransistor circuits that rely on closely matched transistors for optimaloperation, including differential matching circuits, analog amplifyingcircuits, and many digital circuits in widespread use such as SRAMcells. Variation can be even further reduced by adoption of structuressuch as a screening region, an undoped channel, or a threshold voltageset region as described herein to further effectively increase headroomwhich the devices have to operate. This allows high-bandwidth electronicdevices with improved sensitivity and performance.

In summary, a dual-screen DDC transistor is disclosed. There is provideda transistor device having a gate, a doped source and drain region oneither side of the gate and embedded in the substrate, for which thesubstrate comprises a substantially undoped epitaxial layer (prior tothe formation of the source and drain regions), a first heavily dopedregion doped with dopants of opposite polarity as the source and draindopants, the first heavily doped region recessed a vertical distancedown from the bottom of the gate at a depth of 1/1.5 to ⅕ times the gatelength, and a second heavily doped region adjacent to the first heavilydoped region, wherein the second heavily doped region is also of theopposite polarity as the source and drain dopants, the second heavilydoped region which may have a higher or lower concentration of dopantsthan the first heavily doped region and may abut the first heavily dopedregion. In addition, there may be one or more separately doped regionsalso of the opposite polarity as the source and drain dopants to serveas anti-punch through. Variations in the location, number of regions,and dopant concentrations allow for a substrate to include multipletransistors with differing threshold voltages.

Although the present disclosure has been described in detail withreference to a particular embodiment, it should be understood thatvarious other changes, substitutions, and alterations may be made heretowithout departing from the spirit and scope of the structures andmethods disclosed herein. Numerous other changes, substitutions,variations, alterations, and modifications may be ascertained by thoseskilled in the art and it is intended that the present disclosureencompass all such changes, substitutions, variations, alterations, andmodifications as falling within the spirit and scope of the structuresand methods disclosed herein. Moreover, the present disclosure is notintended to be limited in any way by any statement in the specification.

What is claimed is:
 1. A method of fabricating a semiconductorstructure, comprising: concurrently implanting in a substrate a firstscreening region for each of first, second, and third transistorelements; implanting in the substrate a second screening region abovethe first screening region for the second transistor element; implantingin the substrate a third screening region above the first screeningregion for the third transistor element; forming a substantially undopedepitaxial layer covering the first, second, and third screening regionsto form a channel layer for each of first, second, and third transistorelements, wherein the first screening region is in contact with at leastone of the second and third screening regions.
 2. The method of claim 1,wherein the first screening region is separated from at least one of thesecond and third screening regions.
 3. The method of claim 1, wherein adopant concentration of the first screening region is higher than eachof a dopant concentration of the second screening region and a dopantconcentration of the third screening region different from the dopantconcentration of the second screening region.
 4. The method of claim 1,wherein the third screening region is implanted at a lesser depth thanthe second screening region.
 5. The method of claim 1, wherein: thechannel layer on the substrate for each of the first, second, and thirdtransistor elements is formed by a blanket epitaxial growth.
 6. Themethod of claim 1, further comprising: concurrently implanting a firstantipunchthrough region for each of the first, second, and thirdtransistor elements; concurrently implanting a second antipunchthroughregion above the first antipunchthrough region for each of the first,second, and third transistor elements; wherein the first and secondantipunchthrough regions underlie the respective screening regions ofthe first, second, and third transistor elements.
 7. The method of claim6, wherein the first antipunchthrough region is formed in contact withthe second antipunchthrough region.
 8. The method of claim 6, whereinthe second antipunchthrough region is formed in contact with the firstscreening region.
 9. The method of claim 1, wherein each of thetransistor elements has a gate with a source and a drain on either sideof the gate, each of the first and second screening regions extendslaterally across the length of the channel and abuts the source anddrain.
 10. The method of claim 9, wherein each of the first and secondscreening regions is located to be below the surface of the substrate ata distance of at least less than 1/1.5 times a length of the gate andabove the bottom of the source and drain to which each of the screeningregions abuts.